Aquaculture production systems and technological facilities can be classified based on a [621363]

Aquaculture production systems and technological facilities can be classified based on a
multitude of aspects, of a technic, technological and ecological nature. It is known that the main
technological facilities encountered in an aquaculture production sy stem are the rearing units,
and these are classified into the following categories:
• Ponds (diked, excavated, dammed);
• “Raceways” – canal -like, long rearing units, with an intense water flow (based on the
design, placement and water flow, these are the fol lowing types: single water passage
“raceways”, parallel or in -series “raceways”, floating/”in pond” “raceways”);
• Tanks of different shapes (circular, octagonal, rectangular) made of concrete, fiber glass,
plastics, metal, etc.;
• Mesh holding structures (flo ating cages, enclosures/pens);
• Shellfish aquaculture specific installations (floating pontoons, floating grates, floating trays,
etc.).
Regarding production systems, the literature mentions numerous criteria for classification.
➢ By type of rearing unit with in the production system:
• Pond aquaculture;
• “Raceway” aquaculture;
• Recirculating aquaculture systems (RAS);
• Floating cages aquaculture;
• Mesh pens aquaculture.
➢ Relative to the rearing intensity level:
• Extensive, semi -intensive;
• Intensive;
• Very intensive.
➢ By the placement of the technological facilities:
• Terrestrial;
• Aquatic;
• Transition.
➢ depending on the complexity of technological management and water quality control the
production systems are:
• Opened, semi -controlled (ponds, “raceways”, mesh retention struc tures: cages and
pens);
• Controlled (systems with serial water reuse, systems with partial water reuse,
recirculating systems).

➢ after the production -environment system relationship, respectively the possibility of impact
control of the production system on the environment:
• Opened (mesh retention structures, shellfish aquaculture specific installations);
• Semi -closed (ponds, “raceways”, other tank -type rearing units);
• Closed (recirculating systems).
➢ according to the way of water management in a production syst em:
• flow-through aquaculture systems (the water passes once through the rearing units and
it is entirely discarded in the waterway from whence it was collected);
• Partial water reuse aquaculture systems – PRAS (a part of the rearing units effluent is
retained and recycled to reuse the water);
• Recirculating aquaculture systems – RAS (the production system effluent is entirely
recycled and reused).
➢ By water salinity:
• Freshwater aquaculture systems/continental aquaculture;
• Saltwater aquaculture systems/marine aquaculture;
• Brackish aquaculture systems/brackish aquaculture.
➢ By the reared species:
• Cyprinids aquaculture;
• Salmonids aquaculture;
• Sturgeon aquaculture;
• Mollusks aquaculture;
• Crustaceans aquaculture;
• Algae aquaculture, etc.
➢ Considerations related to the profitability of aquaculture and/or reduction of the
environmental impact has led to its integration/association with different plant and animal
systems. As such, the following production systems can be identified:
• Aquaponics systems;
• Multi -trophic integr ated aquaculture systems;
• Partitioned aquaculture systems;
• Plant growing and aquaculture integrated systems;
• Animal rearing and aquaculture integrated systems;
• Substrate -based aquaculture systems;
• Periphyton -based aquaculture systems.
Thus, the industrial aquaculture activity operational management differs mainly depending
on the production system that is being used.

1. Recirculating aquaculture systems SWOT analysis

STRENGTHS WEAKNESSES
• Very low environmental impact;
• Improved product bio -security and food safety;
• Great water conservation;
• Improved control over discharge/effluent;
• No issues with predatory species;
• No issues with escapees;
• Excellent temperature control;
• No (or very low) use of medicine;
• No weather issues;
• Efficient constructed space use;
• Close to market;
• Continuous production, all year round;
• Safer working conditions;
• Ability to sustain very high stocking densities
in a much smaller volume of water. • Higher implementation costs;
• Higher electricity costs;
• Evolving system design;
• Not labeled as “organic”;
• Poor investor confidence;
• Intensive rearing bad image;
• Profits are marginal compared with other
aquaculture systems;
• Issues of flesh tainting;
• Rearing knowledge limited to only a few
species;
• Need of experienced staff to run/maintain the
system;
• Limited marketing experience;
• Higher sensitivity to changing market prices.

OPPORTUNITIES THREATS
• Production system automation;
• Reduction of capital costs and
running/operating costs;
• Excellent integration with alternative energy
sources;
• Safe for rearing new species;
• Excellent management and record keeping of
the production system;
• Availability of backup and alarm systems;
• Improved marketing and product placement;
• Stock improvement and genetic selection;
• Reuse of waste as fertilizer/nutrients for
aquaponics systems;
• Consumer demands for healthy products. • Diseases, and the fast spreading of this due to
the higher stocking density in a smaller water
volume;
• Possibility of increasing energy costs;
• Market susceptibility to cheaper imports;
• Fish feed cos ts and availability;
• Human error;
• System component failure;
• Power failures (in case of nonexistent backup
power source);
• Low/poor quality systems/system components;
• Scalability issues;
• Availability of veterinary medicines;
• Availability of experienced perso nal.

2. Examples of configurations for recirculating integrated systems which
uses aquaponics techniques

A. First design (Recommended for hobbyist/family production systems)

Figure 1 . Recirculating integrated aquaculture system with for hobbyist/family use

B. Second design (Recommended for small scale production systems)

Figure 2. Recirculating integrated aquaculture system with aquaponics module placed after
the biological filtration unit
C. First design (Recommended for medium scale production systems)

Figure 3. Recirculating integrated aquaculture system with aquaponics module placed
above fish growing units and communicating directly with them.
D. Third design (Recommended for large scale production systems )

Figure 4. Large scale recirculating integrated aquaculture system with aquaponics module
placed after the biological filtration unit

3. Processes in recirculating integrated aquaculture systems, based on
aquaponics techniques

4.1. Sediments removal

4.1.1. Introduction
Suspended solids removal represents a primary objective in the design of water treatment
schemes within recirculating systems. Numerous organic particles that can be found in the rearing
environment strongly influence the noxious substances content and the oxygen content, leading
to serious diseases of the biological material within the rearing systems, especially in the case of
those with a high level of water reuse.
The impact caused by the suspended solids within the water consists in the following:
causing lesions to fish, mechanical clogging of the biological filters, increasing ammonia levels
due to the nitrification processes, and the increase in oxygen consumption needed for organic
matter decomposition. As a consequence, solid wastes are an importa nt factor in limiting

productivity, especially in the case of systems those with a high level of water recirculation. The
EIFAC recommended maximum allowed concentration for solid particles content is 15 mg/L.
There are three ways of approach in regard to the suspended solids control within
recirculating systems:
• Sedimentation and micro -screen filtration – insures the removal of large particles, but are
inefficient for fine particles (< 50 µm); are recommended for systems that function with limited
water reuse;
• Granular media filtration – is efficient for the control of suspended solids under 50 µm and is
recommended for systems with a high degree of water reuse or for systems with special
demands regarding water transparency; just like the micro -screens, the granular media filters
require a small space, but, in general, lead to a much higher loss of hydr aulic load than in the
case of settling tanks (sumps) or micro -screen filters;
• Special physical and chemical processes – consist in the use of special inst allations
(hydrocyclones, activated carbon filters, foam fractionators, ozonizers, porous filters); these
are not used separately, they, usually, constitute complementary filtration to the processes
previously presented.
Within a recirculating system, the total suspended solid (TSS) particles with a diameter
greater than 1 µm, are an important parameter for environment quality assessment. The
concentration of these TSS must be maintained, through an adequate management, within
optimum limits for each reared species. For example, for salmonids, it is appreciated that fine
particles (5 -10 µm) accumulation can have lethal effects.
From a chemical point of view, the suspended solids are both of an organic and inorganic
nature. The organic part, known as volatile suspended solids (VSS) determine oxygen
consumption. The inorganic part, contributes to the forming of sludge deposits, with negative
effects on the habitat.
From a physical point of view, the suspended solids can also be categorized as sedimentable
(> 10 0 µm) and unsedimentable (< 100 µm). As the size of the particles decreases, the efficiency
of the removal processes is reduced.
The removal of the suspended solid particles within an aquaculture production system is
realized through known processes of sol id/liquid phase separation, namely gravitational
separation, mechanical filtration and flotation.
The gravitational separation is based on the sedimentation principle in settling tanks (sumps)
or from case to case in hydrocyclones.
The mechanical filtratio n consists in the retention of the suspended solid phase as water
passes through screens, granular media or porous media.

Through the flotation procedure, the solid particles are attached to the surface of air bubbles
introduced within the water under pres sure and later removed from the system through specific
processes.
In all these processes, the solid phase separation consists in the passing of these particles
through a separation surface. The separation surfaces are represented, according to the
previou sly presented processes, by the bottom of the settling tank, the external surface of the
filtration medium, respectively by the air bubbles.
4.1.2 . Gravitational separation
Gravitational separation consists in the gravitational removal of the solid particl es from the
water of an aquaculture production system. There are three gravitational separation procedures,
namely: sedimentation, centrifugation and hydrocyclone separation.
Sedimentation
This represents the simplest method used for solid macroparticles control. The procedure
consists in the passing of the suspended solids loaded water, at a low flow rate, through a
specially set tank (sump) with the purpose of decanting it.
The settling tanks can function in a continuous or discontinuous regime, dependin g on the
degree of suspended solids accumulated in the water, the granulometric composition of the solid
particles, and the technologic flow rate. For RAS, continuous flow rate settling tanks are most
common.
Continuous flow rate settling tanks, usually re ctangular in shape, are com partmented in four
zones (fig. 5 ) each of these having specific functions regarding the access of technological “dirty”
water, assuring the settling conditions, accumulation of the settled solids and evacuation of the
technologic al “clean” water .

Figure 5 . Continuous flow rate settling tank (Timmons et al., 2002)
The inlet zone assures an even distribution of the suspended solids along the entire surface
of the tank. The settling zone, through its morpho -dimensional characteristics, insures the actual
solid phase separation process through settling. From the sludge accumulation zone, solids are
removed periodically. The settled water is collected at the tank outlet zone level, over its entire
cross -section and is disc harged through the top.
Centrifugation
Centrifugation is a procedure that, mainly, consists in the separation of solid particles from
a system with the help of centrifugal forces induced by the system’s rotation around its own axle.

In this way, the separa tion process is substantially intensified, the dimensions of the solid
particle control system and the time needed for suspended solids removal being considerably
reduced.
From a constructive and functional point of view, the centrifugation separation ins tallations
can have a continuous or intermittent flow rate. For the treatment of aquaculture wastewater, the
continuous flow rate centrifuges are used (fig. 6) .

Figure 6 . Conical scroll -type continuous centrifuge (Wheaton, 1985)
The parameters that influe nce the functioning and the efficiency of the centrifugal installation
are the rotation speed, solid material (TSS) concentration, the system’s clean water discharge
section size, and the thickness of the water layer maintained along the wall of the instal lation. The
clean water outlet section is adjustable according to the degree of solid wastewater loading and
the TSS concentration required for the cleaned water.
Hydrocyclone separation
Hydrocyclones also function on the principle of centrifugal separatio n of the suspended solid
particles in a liquid medium as a result of amplification of the weight difference between liquid and
solid phase under the action of centrifugal force. The separation efficiency is determined by the
difference between solid partic le density and water density.
The figure (fig. 7 ) shows the design schematic of a continuous flowing centrifugal installation
and the principle of its operation. It is noticed that wastewater penetrates tangentially at a certain
speed at the top of the ins tallation. The specific way of the inlet water determines the general
vertical spiral motion of the water in the installation.
The circular motion of the water causes the heavier solid particles to move toward the
hydrocyclone wall. The specific spiral cen trifugal motion of the mass of water within the
hydrocyclone causes a downward current in the peripheral area adjacent to the walls and an
upward current in the central area. Thus, the suspended solids, centrifuged in the wall area, will
be trained towards the base of the hydrocyclone and discharged.
The efficiency of a hydrocyclone used to control solid particles in a recirculating system
depends on its constructive characteristics, the speed of inlet water in the hydrocyclone and the
suspended solids (TSS ) concentration in the water.
Hydrocyclones, relatively small size equipment, are widely used in industry due to their low
cost. The main drawback, which restricts their use, is the high electricity consumption needed to
drive pumps that have to provide hi gh pressure and high water speeds in the installation.
Figure 7. Operation of
hydrocyclone (Wheaton, 1985)

4.1.3. Mechanical filtration
Mechanical filtration is a basic process used to control solid particles in recirculating systems. The
mechanical filters separate the solid phase from the liquid phase when water passes through a
filter medium based on the difference between the particles size of the solid phase components.
Mechanical filters provide an easy way to operate and are relatively easy to maintain under
the conditions of careful design and exploitation. Mechanical filters can be made in various types
of variants depending on the capacity of the growing systems and the loading degree of solids in
water (TSS).
Mechanical filtration does not e nsure total removal of solid particles, very small ones are to
be eliminated by applying specific procedures.
Operating costs of mechanical filters are appreciable if the concentration in the TSS is high,
which is why mechanical filtration must be preceded by a gravitational separation.
Depending on the nature of the filter medium, the following mechanical filtration processes
are most commonly used in aquaculture:
• screen filtration;
• granular media (GM) filtration;
• porous media (PM) filtration.
Screen filtr ation
The process consists in the passage of wastewater through a site system that retains and
eliminates most of the solid particles. The screens are sized according to the solids water loading
(TSS) and granulometric composition.
The main advantage of me chanical screen filter is the recording of low load losses at the
passage of water through the filter medium comparable to those recorded for gravitational
separation.
Mechanical screen filters also show some inconveniences that must be known for a careful
exploitation. Thus, small particles cannot be retained, requiring removal by other processes. Also,
at a high water flow through the filter, large particles fragment, resulting in fine particles not being
retained by the filter. Theoretically, it is possi ble to remove these fine particles if a proper mesh
size is chosen. However, the use of fine webs is limited by some impediments, such as high
pressure losses and rapid clogging of filters. In this case, filter cleaning costs can greatly reduce
the efficie ncy of their use.
Depending on the way of operation and the constructive solution, there are three types of
screens: stationary, rotatory and vibratory.

Stationary screen filters
They are the simplest mechanical filters. In the simplest form, the screen is placed
perpendicular to the liquid flow ( fig. 8 ). In this way, particles larger than mesh size are retained
and collected by it.
Stationary screens are seldom used for particles smaller than 1.5 mm in diameter, or when
the concentration of water in the TS S is too high because there is a risk of rapid clogging in this
case.
The maintenance of stationary sites consists of their periodic removal and their
countercurrent washing with a strong water jet; if the degree of clogging is high and the adhesion
of the solid particles to the screen is strong, the screens are mechanically cleaned with the brush
or by other means.
The screens can be made of various materials resistant to corrosive water (steel, brass,
stainless steel, textile fabrics, and plastics) in a w ide mesh size dimension ranging of microns or
millimeters.

Figure 8. Stationary screen filter (Wheaton, 1985)
Rotary screen filters
Rotary screen filters are designed to reduce the clogging potential which is a major
disadvantage of stationary screens.
Constructively, such a filter is composed of a drum provided with a screen on the outer
surface. The rotary drum is partially immersed in the wastewater which flows through a prismatic
enclosure.
Continuous rotation of the drum causes the immersed part of th e screen to filter solids, and
the upper part periodically passes in front of a mounted flushing system. The washing process,
permanently and automatically, ensures continuous operation, with minimal hydraulic system
losses and low labor need.
Specific to rotary screen filters is the fact that large amounts of water with a significant
amount of solids must be recycled before being discharged from the system.
Depending on the way of water access to the filter section, there are two types of rotary filter
filters, namely axial filters and radial filters.
Axial rotary screen filters
An axial filter with a rotary screen is made up of two chambers, the screen being at the level
of the septum. The wastewater, enters the first chamber, passes axially through the sc reen and
arrives, in filtered form, in the second chamber where it is discharged. The rotation of the screen
causes the partially immersed portion to intermittently face a washing mechanism where a

pressurized downstream water jet washes the retained solid particles; the water loaded with
washed material is taken up by a trough placed on the upstream side of the screen and discharged
(fig. 9 ).

Figure 9. Axial flow rotary screen (Wheaton, 1985)

Rotary screen axial filters are relatively inexpensive, easy to operate and maintain. They
can be washed automatically and efficiently filter out wastewater with a higher concentration of
solid suspension (TSS) than stationary screen filters.
The screens used in an axial rotary filter are circular in shape. This is a major disadvantage
because, given that the water level in the first chamber cannot exceed, for functional reasons, the
axis of the screen, the active surface available for flow is determined by the diameter of the
screen. In the case of high flows, scree ns of appreciable diameters and large filtration enclosures
are required, also more complicated constructive solutions for washing and discharging solids are
required.
Radial rotary screen filters
The radial rotary screen filter consists of a cylindrical d rum that rotates around its horizontal
axis, being partially immersed in filtered water that is passing through a specially designed tank.
The side surface of the drum is the active part of the installation and is a screen.
The suspended solids (TSS) water stream penetrates axially into the drum and is
discharged, filtered, in a radial direction through the mesh network of the screen.
Depending on the system of washing and evacuating solid particles retained in the screen,
there are several constructive var iants of rotary screen radial filters.
Radial rotary screen filters have the same advantages as axial flow filters. In addition, they
are not so restrictive in terms of capacity, as is the case with other filters.
Screens are made from a wide variety of ma terials, from galvanized steel to fabrics. The
choice of the material from which the screen will be made is based on the characteristics of the
treated water, the adopted construction solution and the screen mesh size.

Figure 10. Radial flow rotary screen filter with backwash (Wheaton, 1985)
Chain -type rotary screen filters
Constructively, this type of filter consists of a screen in the form of a funicular strip made up
of an assembly of articulated panels ( fig. 11 ).
The screen is made in articulated varia nt to be rotated by the wheels mounted on the drum
spindle. Since it is difficult to obtain a perfect joint between the screen panels and between the
drive and screen chains, the use of this system is limited, being effective in the case of coarse

material s. The advantages of this type of filter consist in the fact that the size of the filter surface
is appreciable and the operating costs are reasonable. The functional characteristics of chain –
type rotating screen filters recommend them to equip the water i ntakes of aquaculture rearing
systems.
In most constructive solutions, the cleaning of rotary screens is accomplished by continuous
flushing with a water jet. For this reason, water losses used for washing can reach significant
costs, which is a major drawback of these types of filters. Rotary screen filters are recommended
for retaining small solid particles up to 3 μm. The minimum size that can be retained under
effective conditions is limited by the complexity of the plant and the amount of water required for
washing. The yield of the filter is determined by the mesh size, the size of the active surface, and
the amount and characteristics of solid particles in wastewater.

Figure 11 – Chain -type movable screen filter (Wheaton, 1985)
Vibratory screen filters
Vibratory screen filters are special installations where liquid and solid phase separation is
carried out on filter surfaces to be printed in order to intensify the filtration process, a vibrational
movement in the horizontal plane.
Depending on the direction of the water inlet into the inst allation, the vibratory screens are
of two types: axial drainage and radial drainage. The most commonly used in the aquaculture of
the recirculating systems are those with axial discharge (fig. 12 ).

Figure 12 – Axial flow vibratory screen (Wheaton, 1985)
Wastewater loaded with solid particles moves at a certain speed along the vibratory screen
where the separation of the two phases occurs which are collected and discharged from the
system. The vibration motion is performed with electromagnetic vibrators ha ving amplitudes and
fixed or adjustable frequencies that are determined by the concentration and characteristics of
the suspended solids.
In the case of axial vibratory filters, it is important to ensure an optimal correlation between
the feed rate and the kinematics of the site so that the moisture content of the retained and
removed solids is small enough to reduce the water loss in the system as much as possible.
Granular media filtration
This type of filtration involves passing the waste water stream th rough a layer of granular
material (medium) and retaining the solid particles on its contact surface; the most commonly
used granular material is sand, but under certain conditions, other filtering agents (most often
plastic floating beads) may be used. Gr anular filters work gravitationally or under pressure and
the direction of water circulation through the filter medium can be upward or downward ((fig.13) .

Sand filters
Sand filters consist of a layer of sand or other mineral granular material (gravel) th rough
which water passes. Filtration is a mechanical process that results in retaining solid particles at
the surface or pores of the filter medium, depending on their size .
The maximum particle size that can be retained in the filter is conditioned by the size of sand
grains generally ranging from 2.0 to 0.02 mm. In order to ensure the retention of smaller particles
in the order of the microns, very fine grained clay, clay or similar materials may be used; In this
case, the flow rate of the water through t he filter medium is very low, reducing the efficiency of
the filter. As a rule, sand filters completely remove only particles larger than 30 μm in diameter.

Figure 13. Gravity flow sand filter (Wheaton, 1985 )
The discharge rate through the filter and its clogging intensity are dependent on the particle
size of the filter medium and the concentration or characteristics of the solid particles in the
wastewater. Filtering media with fine grain yields low drainage speeds. The higher the
concentration of TSS, t he faster the filter will clog, and more frequent washing and, implicitly, high
washing water demand will be required.
Sand filters can work gravitationally or under pressure; the choice of one of the two functional
types is based on the concentration of the TSS in the waste water and the t echnologically clean
water flow (fig.14).

Figure 14 Pressure sand filter (Wheaton, 1985)
Bead filters
The filtration medium for these filters is a layer made up of 3 to 5 mm diameter plastic floating
beads. Due to the subunit specific weight, the plastic, lighter than the water, forms a compact bed
at the top of the filtration chamber which, by complex adsorption processes, retains the solid
particles in the wastewater.
Depending on the TSS water load and the constructi on features of the growing system, the
bead filters can operate free or under pressure and the water flow direction in the filter can be
descending or ascending.
The size and density of the beads are determined by the nature of the solid particles and
the concentration in the TSS.
The plastic bead filters have the advantage of a high reliability and the possibility of applying
some easy washing procedures.
Porous media filtration

Porous media filters are used to remove smaller solid particles that cannot be effectively
retained by other processes.
The operating principle of these filters is similar to that of the screen filters and consists in
passing the waste water through a microporous structure filtering medium in which suspended
solids are retained. Unl ike screen filters, porous agents are characterized by lower filtering speeds
and higher pressure losses.
In porous media filters, the thickness of the filtering layer is higher than that of the screen
filters, and the pore size is smaller than for regular granular filters.
The most commonly used porous agent is diatomaceous earth (DE).

Figure 15 . Diatomaceous earth filter system (Wheaton, 1985)

Diatomaceous earth filters (fig. 15 ) are mechanical filters that are used for rearing systems
that require a hi gh technological requirement in terms of clarity and degree of microorganism load
in the water. There are a multitude of diatomite earth categories used as filter media, the finest of
which is capable of retaining solid particles with diameters up to 0.1 μ m.

4.1.4 Physico -chemical processes
The physico -chemical processes accomplish the retention of fine solids as well as dissolved
solids, mainly through adsorption processes. Adsorption can be defined as a process of
accumulating or concentrating a substance on a separation between two phases. In waste water
treatment, adsorption usually takes place at the level of the separation between a liquid and a
solid or the separation surface between a liquid and a gas.
The most commonly used methods for treating waste wa ter in recirculation systems by
physico -chemical processes are: adsorption at the level of the separation surface between a liquid
and a solid (active carbon filters and resin ion exchange filters) and adsorption to the separation
surface between a liquid and a gas (foam separator).
Active carbon filtration
The process consists in the passage of waste water through an active carbon layer where
adsorption resolves the dissolved substances. The dynamics of the adsorption process is directly
proportional to th e decrease in surface tension at the level of the solid -liquid separation surface,
which in turn is directly proportional to the concentration of surfactants represented by the solutes
dissolved in water.
The efficiency of chemical filtration depends mainly on the size of the contact surface
available for the adsorption process. Active carbon is the material that satisfies this demand. The

carbon activation process consists in the formation of a very large number of cracks in its structure
by specific (thermal or chemical) processes, to which corresponds a significant contact surface .

Factors influencing adsorption on carbon
Adsorption on carbon depends on many factors whose influence is often simultaneous and
difficult to quantify. The main factors that influence this process are: the size of the contact
surface, the solvent's characteristics, the water reaction, the water temperature, the degree of
diversity of the solvents.
• The contact surface is the most important factor influencing the adsorption process. As a
surface phenomenon, the adsorption level is determined, as mentioned, by the size of the
contact surface available to the adsorption process. The size of the contact surface depends
on the starting material from which the activated carbon is obtained, the type of activation
process used to produce it and the size of the constituent particles. Thus, activated carbon
can be obtained from various materials: bones, coal, wood, walnut shells (especially coconut
and peanuts), sawdust, processing waste and agricultural waste.
• The solubility properties, the molecular weight and the ionic characteristics are the
characteristics of the solvent that influences the adsorption dynamics (as velocity and
efficiency ). The law of Lundelius, the first of the qualitative laws defining the influence of
solvation characteristics on the adsorption process, establishes that the degree of
adsorption of the solution in a solution is inversely proportional to its solubility.
• The water reaction (pH) frequently influences the adsorption of certain ions, partly because
the hydrogen and hydroxide ions tend to be strongly adsorbed, and on the other hand
because the pH influences the degree of ionization of the acid solutions and bas ic. The
ionization state of these solutions influences adsorption. In general, organic pollutants in
water are better adsorbed to lower pH.
• The effects of the temperature on the adsorption process are both direct and indirect. Since
the adsorption process is exothermic, low temperatures tend to favor higher adsorption
rates. Indirect influences refer to the influence of temperature on viscosity and water density.
However, the temperature in ordinary aquaculture systems has a relatively insignificant
influen ce on adsorption on activated carbon.
• The degree of diversity of solvates in water from aquaculture systems is also a factor
influencing the dynamics of carbon adsorption. It has been shown that the presence of more
than one dissolved in water decreases th e adsorption rate of each of them, but increases
the total adsorption capacity of the active carbon over the value that would have been
achieved in the case of a single solvite substance.
Types of activated charcoal filters

From a constructive and function al point of view, charcoal filters can be intermittent or
continuous.
The activated charcoal filter with intermittent flow regime consists of a granular activated
charcoal tank flooded with wastewater. Typically, the water -coal mixture is agitated to speed up
the adsorption process. For a specified period of time, called contact time, the water -charcoal
mixture is left at rest, after which the purified water is drained and the sedimentary coal is either
removed or reactivated. Active charcoal filters with i ntermittent flow mode function properly for
relatively small systems. It has the disadvantage of requiring high operating expenses to have a
limited flow.
Active charcoal filters with continuous flow mode can be classified according to flow direction
in as cending flow filters and descending flow filters.
The first system uses a granular activated charcoal pool similar to the intermittent system.
Waste water, introduced continuously at the bottom of the basin, passes through the charcoal
bed and is discharge d to the top of the tank. In this case, contact time is dependent on the ascent
water speed as well as on the height of the coal bed. Upward flow filters can use either granular
or powdered charcoal.
The second type of continuous flow filter exposes an act ivated charcoal bed (usually a
column) to a descending stream of wastewater. This system eliminates oversized basins and is
frequently used to treat municipal waters.
Filters with ion exchangers
Ion exchange is an electrochemical process that consists of exchanging ions between two
substances, usually a solution and an insoluble solid in that solution. Ion exchange is not only an
adsorption phenomenon on a surface, but also involves the three -dimensional internal structure
of solid phase molecules. Ion exc hange is, in these conditions, a complex process of both
adsorption and absorption.
Ion exchange resins can be classified, depending on the mode of production and their
characteristics, into one of the following four groups: very acidic cations, weak acid cations, very
basic anions, weak basic anions. There are also some materials (clays and zeolites) in which
ionic exchange occurs naturally. Choosing the right resin to retain a particular component
depends on its properties.

Foam Fractionation (separation )
Foam fractionation is a process of separating or concentrating the dissolved materials by
adsorbing one or more solvents to the surface of the air bubbles passed through the solution to
be treated. Thus foam forms on the surface of which are dissolved bo th dissolved substances
and solid particles (TSS) which are fixed on the surface of the bubble.

By removing the foam from the surface of the liquid, solvates and solid particles
concentrated in the respective area are eliminated at the same time.
There are several separation techniques based on foam fractionation, some of which apply
with good results in aquaculture, such as bubbling and flotation.
Bubble fractionation is similar, with respect to the operating principle, to the foam
fractionation process wh ich differs in that no foaming occurs. Bubble fractionation occurs where
a surfactant is present but, for various reasons, the solution does not foam. As the solution near
the surface of the liquid is enriched with solvent, removing the surface fluid leads to a decrease
in the solvent content in the waste water introduced at the bottom of the column.
Flotation -based processes are used for waste disposal, ore purification and microorganisms
concentration. These processes consist of bubbling, under a certain pressure, the air into the
wastewater which, from a physical point of view, can be considered a liquid -solid mixture. In this
case, the air bubbles attach to the solids, the process continuing until the solid – bubble
combination has a specific apparent we ight less than that of the water, at which point the solids
become floating and can be easily removed from the system. The physical principle of flotation
consists in the transfer of dissolved substances and fine solid particles from wastewater by
adsorpti on to the gas bubbles.
The operating principle of the installation (fig. 16) consists in pumping the waste water
(solvent and solvite) into a circular section column, where air is introduced as a small bubble
through a diffuser tube at the lower part.

Figure 16 Basic foam fractionation unit (Wheaton, 1985)
In their ascension move, the bubbles collect the solution they concentrate on the surface of
the water. Thus, at the surface of the liquid the bubbles pass into the foam phase, carrying with
them the sol vite charge and fine solid particles together with a small amount of solvent. The
process of continuous foam formation causes it to be lifted into the column until it is forced to pass
into the collector from where it is discharged.
The filtered water is c ollected at the base of the column and the level of the liquid in the
column is controlled by a tap placed on the path of the filtered water outlet .

Ozonation
Ozonation is not a proper procedure for separating solvents from wastewater. Ozone helps
to control solid particles indirectly by modifying particle size. Ozone, a highly unstable reactive
gas, has the ability to fragment high weight molecular organic substances in to simpler substances
that can be readily biodegraded or retained by precipitation or adsorption.

Ozone is used in many aquarium recirculation systems to reduce turbidity and water color.
The effect of ozonation on the change in solid particle size is not yet clearly defined.
Particular attention should be paid to residual ozone in water, which may cause dysfunctions
at the gill level and, in certain situations, even the death of fish or other organisms coming into
contact with ozonated water.
For these rea sons additional research is needed before recommending the use of this
procedure for controlling solid particles in aquaculture recirculation systems .

4.1.5. Applications for solid particle control systems

The main criteria in choosing a particular process for TSS control in a recirculating system
is to ensure an optimum water quality with respect to the suspended solids content with minimum
capital and running costs.
Typically, the processes and installations presented are integrated into complex
technological schemes that satisfy the above mentioned need.
A complete technology and related facilities for TSS control are shown schematically in
fig.17 . Depending on the particle size and concentration, the applied technology may comprise
all three phase s (pretreatment, main treatment and finishing treatment) or only a part thereof.
The most used pre -treatment installations are sedimentation basins. The main problem to
be considered when applying this process is the limited possibility of removing fine pa rticles and
the relatively high space requirement.
Also, it is necessary to know that with the removal of sludge from the pre -precipitator a
significant solubilization of the suspended solids occurs in the water mass, contributing to the
degradation of the water quality in the system. Therefore, sedimentation pretreatment is a
recommended procedure for resistant species.
Pretreatment of water in tubular decanters has the advantage that it greatly reduces the
technological space and limits the water loss to an appreciable extent, compared to the usual
sedimentation basins. However, the high degree of accumulation of fine particles and, implicitly,
the degradation of recirculated water constitutes an important restrictive factor for the applicability
of these technologies. Biofilter filling, capable of reducing organic matter loading, makes it
possible to use these separators to pre -treat waste water in a recirculating system.
The efficiency of solid particle separation is obtained by associating the pretreatme nt
processes with the actual treatment processes. Among the main treatment processes
encountered in the practice of recirculating systems, most are based on granular media (GM)
filtration.

GM filters are used after the removal of coarse particles by sedime ntation, so that the
wastewater reached in this treatment stage has a relatively low concentration of TSS. This avoids
the clogging of the filters, reduces the frequency of backwashing and, implicitly, the water and
energy consumption associated with these operations. Combining pre -settling with the proper
filtration with granular agents ensures optimal wastewater treatment under conditions of
significant TSS associated with high biomass density in aquaculture rearing systems.

Figure 17 – Solids removal p rocesses and the particle size range in micron over which the
processes are most effective (Timmons et al., 2002)

The upward flow filters eliminate the clogging and compaction problems that occur in filters
where the flow is descending. The downside of th e upward filters is that it involves a difficult and
long-lasting, more expensive cleaning. Wet sand filters partially solve the systems with moderate
load in the TSS, the deficiencies mentioned above.
For fish species susceptible to TSS concentration, or for some more sensitive stages of their
development, a final treatment of waste water, namely “finishing”, is required.
Filters with porous media (PM) and different physicochemical processes can be used for this
purpose, the quality of treated water justif ying the high cost of these equipment.
Knowing the mechanisms of each solid particle removal process is essential to designing
integrated treatment schemes that meet the requirements of each rearing technology.
The wastewater treatment scheme for a recirculating system is primarily conditioned by its
carrying capacity, implicit by the nature and level of concentration in the TSS.

4.1.6. Ratings of different mechanical filters
System
technological
process /
component Technology /
Equipment Sub-technology /
Sub-equipment Space
requirements Operational
management
(Ease of use) Operational
performance Acquisition and
implementation
cost Maintenance
and operation
costs
Mechanical filtration Gravitational
separation Sedimentation / Sump     
Centrifugation / Centrifuge     
Hydrocyclone separation /
Hydrocyclone     
Mechanical
filtration Screen filtration / Stationary
screen filter     
Screen filtration / Axial rotary
screen filter     
Screen filtration / Radial rotary
screen filter (Drum filter)     
Screen filtration / Chain -type
rotary screen filter     
Screen filtration / Vibratory
screen filter     
Granular media filtration / Sand
filter     
Granular media filtration / Bead
filter     
Granular media filtration /
Porous media filter     
Physico –
chemical
processes Active carbon filtration /
Diatomaceous earth filter     
Ion exchange filter     
Foam fractionator     
Ozonator     
NOTE: Regarding the scoring system: Space requirements (where  is large and  is small); Operational management (Ease of use – where  is hard and 
is easy); Operational performance (where  is low and  is high); Acquisition and implementation cost (where  is expensive and  is cheap); Maintenance
and operation costs (where  is expensive and  is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).

4.2. Biological Filtration

2.1. Introduction
Like all living organisms, fish require a clean environment for optimal growth and
survival. As fish respire and metabolize feed, toxic metabolites are released into the water
column. Metabolite accumulation increasingly degrades system water quality. If i norganic
or organic toxins within the water surpass biologically critical levels, fish growth may
become inhibited and mortality increased.
To maintain a clean environment in recirculating systems, a combination of
mechanical and biological filtration tec hniques must be employed. Although nitrification
can occur throughout the culture system (e.g., in biofilms on pipe and tank walls)
(Losordo, 1991), the majority of biochemical reactions pertaining to heterotrophic and
autotrophic bacteria occur within bio filters. Biofilters are specifically designed for
concentrated bacterial attachment and nitrification via fixed -film processes.

4.2.2. Biofiltration and nitrification
Autotrophic bacteria are credited for performing nitrification (Wedemeyer, 1996).
Nitrification is a two -step process, where Nitrosomonas sp . oxidize ammonia to nitrite,
and Nitrobacter sp . oxidize nitrite to nitrate. Although less toxic than ammonia, nitrites
also are considered toxic to fish, while nitrates (NO 3–N), the final oxidized form in
nitrification, are considered relatively nontoxic to fish unless high concentrations are
sustained for an extended period of time (Spotte, 1979), (fig.18). Since biofiltration is the
principal unit process used for treating fish metabolites, biofilters can be considered major
components in intensive recirculating aquaculture systems (Libey and Miller, 1985).
To ensure prolonged fish survival, high levels of sustained nitrification must be
achieved. Therefore, ecological requirements of the bacteria (Mal one et al., 1993) must
be met within biofilters for effective nitrification to occur. System water quality and filter
design characteristics affect filter environmental conditions. Although a larger number of
water quality parameters affect nitrification k inetics, Kaiser and Wheaton (1983) stated

that dissolved oxygen, pH, water temperature, ammonia -N concentrations, and filter flow
rate are the dominant factors affecting a filter’s nitrification efficacy .

Figure 18. Nitrogen cycle
Nitrification Denitrification
1. Heterotrophic bacteria; 2. Nitrosomonas sp .; 3. Nitrobacter sp .; 4. Plants

4.2.3. Configuration of the nitrification filter
There is a variety of nitrification filters in aquaculture. Depending on the construction
solution and the operating mode, the following types of nitrifying filters are distinguished:
submersible, drum, disk, fluidized bed and sand.
Submersed (submerged) filters
The sketch of a submerged filter is shown in fig. 19 . Its operatin g principle consists
in the passage of water through a filtering layer made up of different materials, the usual
mineral aggregates (sand, gravel, broken stone) or granular plastic structures. A
distinctive feature of these filters is that the filtering ag ent is permanently immersed.
Water flow through the filter can be upward or downward. Since the population of
nitrifying and heterotrophic bacteria is permanently below the water level, the oxygen
required for metabolic activities is represented by oxygen in the water, the atmospheric
being inaccessible. The possibility of exclusive use of dissolved oxygen in water is one of
the main limits of submerged nitrification filters. Operation of the submerged filter involves
making a column of waste water over the filter layer. Filtration speed and filtered water
flow, respectively filter yield, are determined by the height of the water column that causes
water to flow through the filter at a certain speed. When determining the height of the
water column, account s hall be taken of the pressure losses occurring in the filter bed
which, in the case of this type of filter, are relatively small (5÷60 cm/m).

Figure 19. Submerged filter (Timmons and Losordo, 1994)
Trickling filters

The operating principle of these filter s consists in passing water through the filter
medium in the form of drops. Water circulation in the trickling filter is descending, similar
to that of the submerged filter. In contrast, however, the submerged filter where the water
flows into forced mode, the trickling filter circulates gravitationally under the action of its
own weight. As a result of this specific flow of water, the filter agent is not flooded, being
permanently kept wet. Under these conditions, the filter medium is well aerated, providi ng
enough oxygen for the bacterial population. Airflow through the filter provides most of the
oxygen required for the filter (as opposed to saturated water containing up to 15 ppm O 2,
the air contains 210000 ppm O 2). The energy required for the operation of these filters is
mainly represented by the energy needed to pump the water at the heights and the
required technological flows. There are situations when energy is also consumed and for
the introduction of pressurized air into the filter medium by means of compressors.
The thickness of the filter layer on a trickling filter varies between 0.15÷5 m,
depending on the nature of the agent used, the design solution, the flow rate and the
degree of waste water loading.
Periodically, the trickling filters are counter -current washed to eliminate the solids
deposited and restore the physical structure of the filtering agent. The washing frequency
is as low as possible, as with submerged filters, as long as the restoration time of the
bacterial population (biologi cal film) is appreciable (20÷30 days).
Figure 20 presents the constructional drawing of a trickling filter in which water is
uniformly distributed over the filter media.

Figure 20 – Trickling filter (Losordo et al., 1999)
Drum filters (Biodrums)
They con sist of a perforated cylindrical drum disposed in a vat through which waste
water passes (fig. 21). The drum is filled with a certain type of granular agent having high
porosity and/or active surface values. The rotation of the drum is made with an axial s haft.
The water level in the drum ensures immersion of half the diameter of the drum. As a
result of the rotating motion of the drum, the biological film formed on the granular agent
passes alternately, at a certain frequency, both through air and water.

The rotation speed of the drum is such that the duration of the biological film,
represented by the heterotrophic and autotrophic bacteria populations, does not affect its
development due to lack of oxygen. It is also important that the drum speed does not lead
to the maintenance of the biological film longer than necessary to avoid dehydration
(drying) of the biological film. In the drum filter due to the specific mode of operation, the
oxygen content of the water is no longer as important as in the case of the submerged
filter, the concentration in ammonia and nitrite being the one that determines the spin
speed of the drum (a lower content in ammonia and nitrite assumes a lower speed, a
stronger load of water in nitrogen compounds requires a higher rotatio nal speed).
The dynamics of oxygen consumption in the water is different, depending on the
speed of the drum. Thus, in the case of lower rotation speeds, the oxygen in the water is
consumed by bacteria before they are exposed to air. At too high a rate of erosion of the
biological film occurs through the entrainment of bacteria due to the hydrodynamic action
of the water stream subjected to filtration. The rotating motion of the drum causes a
turbulent water regime inside the filter, which is why the filtra ting agent's clogging rate is
quite low. With regard to the energy consumption required for operation, the drum filters
have a specific energy consumption higher than the disk filters because of the higher
turbulence of the water. In a drum filter the load losses along it are usually 1 -5 cm .

Figure 21. Drum filter (Timmons and Losordo, 1994)

Rotating Biological Contactor (RBC)
The operating principle of RBC nitrifying filters is similar to that of drum filters,
namely periodically bringing into contact with air the biologically active film. The active
surface, on which the nitrifying biological film develops, is represented by t he lateral
surface of disks disposed spaced on a rotating shaft at a certain speed, in a continuous
rotation motion (Figure 22). The optimum distance between discs is, as a rule, approx.
20-30 mm. This distance results in spaces between discs that are smal l enough to provide
an active filter surface as large as possible. The aforesaid distance also provides the vital
space necessary for the formation and stabilization of the biologic film on both sides of

each disc and optimum hydraulic conditions for the c irculation of water between the discs
covered with bacteria.

Figure 22. Rotating Biological Contactor (RBC) (Losordo et al., 1999 )
The rotational speed of the RBC filters depends, as with the drum filters, primarily
on the water loading in ammonia and nit rogen and, to a lesser extent, on the oxygen
content of the water.
The turbulence created by the binoculars in the filtering mode of the wastewater is lower
than in the case of the drum filter, and consequently the power required to drive the
shaft, or the specific power consumption that ensures the operation of this type of filter,
are lower than in the case of the rotary drum filter.
For RBC filters, the load losses registered in the water flow direction are almost
inexistent, so RBCs are also more effect ive from this point of view.
When the design of these filters is judicious, especially with regard to the distance
between the disks and the speed of rotation, the clogging rate is very low and the washing
frequency is low.
Fluidized beds filter
From a constructive point of view, these filters consist of a cylindrical basin whose
dimensions (height and diameter) are determined by the waste water flow and its loading
into toxic nitrogen compounds (ammonium and nitrates). In both the lower and the upper
part, the basin is provided with specific reinforcements that provide access to the
wastewater and the effluent. On a perforated septum placed at the bottom of the unit, the
filtering agent is placed, on which the biological film will be formed and developed (fig.
23).

Figure 23. Fluidized beds filter (Timmons and Losordo, 1994)
Medium specific gravity filtering agents are used to easily bring in the float and with
the finest granularity so that the active surface is as large as possible. The filter agent
that corresponds to the highest requirements is fine sand. Waste water is pumped into
the bottom of the filter by pumping pressure. In its ascending motion, due to the

hydrodynamic drive force, the waste water brings and maintains the floating agent in the
form of a fluidized bed. The fluidized bed thickness depends on the physical -mechanical
characteristics of the filter agent and the water velocity. As a rule, the fluidized bed
occupies a smaller or larger part of the filter height. The water velocity in t he filter is set
so that the filter agent is kept permanently suspended in the water mass; at low water
velocities, there is a danger that the filter agent deposits on the perforated septum and at
higher speeds it can be removed from the filter. On the act ive surface of the fluidized bed
a certain biomass is developed which performs the nitrification process. In the case of
fluidized bed filters, the oxygen required for the biological film is supplied exclusively by
the water stream. Under these conditions, it is important to ensure a certain water velocity
depending on its oxygen content. A fluidized bed filter operates efficiently when the
dissolved oxygen concentration corresponds to the saturation limit for a particular
temperature.
The fluidized bed fil ters generally require large flow rates on the surface unit and
are small in size, being compact. It balances hard and once balanced they have good
efficiency. Most of the energy required to ensure their functioning is used for the
fluidization of the filt ering agent and depends on the flow rate and the amount of load
losses.
Bead filter
From a constructive and functional point of view, plastic ball filters are similar to
those previously described ( fig. 24 ). The difference between them lies in the nature o f the
filtering agent, which in this case is represented by plastic balls having a specific subunit
weight, thus lighter than water. For the filter layer to have a specific surface area as large
as possible, the size of the plastic balls is as small as mil limeters (usually 2 -4 mm).

Figure 24. Bead filter (Timmons and Losordo, 1994)
The water flow in the filter is upward. The speed of the ascending stream of water
to a plastic ball filter may be lower than that of the fluid bed filter since its floating co ndition
is given by the specific subunit weight of the beads, and a certain hydrodynamic drive

force or a certain speed is not required minimum limit. The water flow speed in a plastic
ball filter is mainly conditioned by the wastewater flow.
Both at the b ottom and the top, the filter tank has two perforated plate screens,
disposed at different heights. The top perforated plate screen holds the filter balls while
running, while the perforated bottom plate screen forms support for the filter agent when
the filter does not work or when the water flow is discontinuous.
Periodically, plastic ball filters are washed to restore granularity, or the specific
active surface of the filter medium. There are several washing methods. One of these is
to periodically disco ntinue the filter operation and drain it; after restarting, for a short
period of time, the effluent is discharged out of the system due to its high suspension
content. A second method, which does not require the interruption of the filter function,
consis ts in periodically shaking the ball bed with a mechanical device to displace the
excessively developed biological film as well as the retained solid particles; as with the
previous method, for some time the effluent is eliminated outward, the filter being
connected to the growing system when the water is clean.
It results that plastic ball filters provide mainly biological filtration (control of nitrogen
compounds), but also perform a mechanical filtration of solid particles due to the granular
structure of the filtering agent.
The plastic ball bed filters are relatively small in size, so they are more economical
under certain conditions than other biological filters because the small diameter of the
filter agent provides a large active surface.